A light-emitting device using a light-emitting element which includes a layer containing an organic material between a pair of electrodes and emits light when current flows between the electrodes has been developed. Such a light-emitting device has the advantage of being thin and light, compared with other display devices which are now called thin display devices. Such a device also has high visibility since it is a self light-emitting element, and has a fast response speed. Therefore, this kind of light-emitting device has been actively developed as a next-generation display device, and has partly come into practical use.
The layer containing an organic compound provided between electrodes may have either a single layer structure including one light-emitting layer or a layered structure including layers having different functions from each other; however, the latter, a function-separated type layered structure, is often employed. As an example of the function-separated type layered structure, a structure where a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injecting layer are sequentially stacked over an electrode serving as an anode is typical, and each layer is formed using a material specific to each function. Note that a layer having two or more kinds of these functions such as a layer having both functions of a light-emitting layer and an electron transporting layer or a layer having another function such as a carrier blocking layer may be used.
Materials used for these functional layers are required to be materials specific to functions each layer serves and to have high heat resistance, since the heat resistance of the material itself greatly affects heat resistance of the light-emitting element. The materials are also required to be materials which do not adversely affect another layer when forming a layered structure, and research has been carried out to seek better materials. For example, because 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbrev.: TPD) which is conventionally used as a hole injecting material or a hole transporting material has a low glass transition temperature (Tg) of 67° C. and has low heat resistance, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbrev.: NPB), which is formed by substituting a methylphenyl group in TPD for a naphthyl group so as to increase the Tg to 96° C., has been proposed and is widely utilized (for example, Reference 1: S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic electroluminescent devices with improved stability”, Appl. Phys. Lett. 69 (15), 7 Oct. 1996).
However, while NPB has a higher glass transition temperature (Tg), its energy gap is lower. Accordingly, TPD emits light of the violet region, whereas NPB emits light of the blue region. In other words, NPB can be said to be a material that has gained a better Tg than TPD by sacrificing its energy gap. NPB and TPD are often used for a hole transporting layer, and in many cases are provided adjacent to a light-emitting layer. In such cases, if the energy gap of the hole transporting layer provided adjacently is small, there is a risk that excitation energy will be transferred to the hole transporting layer from a light-emitting material or a host material excited in the light-emitting layer. When excitation energy is transferred from the light-emitting layer to the adjacent layer, light-emitting efficiency of the light-emitting element is degraded, or color purity is reduced. Degradation of light-emitting efficiency and reduction of color purity in a light-emitting element cause increase of power consumption and degradation of display quality respectively, in a light-emitting device or an electronic device using the light-emitting element. Therefore, a layer in contact with a light-emitting layer desirably has as large an energy gap as possible.